1. The Midlatitude Cyclone Ahrens, Chapter 12

2. Waves on the polar vortex
Hemispheric westerlies typically organized into 4-6 “long waves”
Wind blows through them, but the waves themselves propagate slowly (east to west!) or not at all

3. A “Family” of Cyclones

4. Fronts A Front - is the boundary between air masses; normally
refers to where this interface intersects the
ground (in all cases except stationary fronts, the
symbols are placed pointing to the direction of
movement of the interface (front)
Warm Front
Cold Front
Stationary Front
Occluded Front

5. Characteristics of Fronts
Across the front - look for one or more of the following:
Change of Temperature
Change of Moisture characteristic
RH, Td
Change of Wind Direction
Change in direction of Pressure Gradient
Characteristic Precipitation Patterns

6. Typical Cold Front Structure
Cold air replaces warm; leading edge is steep in fast-moving front shown below due to friction at the ground
Strong vertical motion and unstable air forms cumuliform clouds
Upper level winds blow ice crystals downwind creating cirrus and cirrostratus
Slower moving fronts have less steep boundaries -- shallower clouds may form if warm air is stable

7. Typical Warm Front Structure
In an advancing warm front, warm air rides up over colder air at the surface; slope is not usually very steep
Lifting of the warm air produces clouds and precipitation well in advance of boundary
At different points along the warm/cold air interface, the precipitation will experience different temperature histories as it falls to the ground

9. The Wave Cyclone Model (Norwegian model)
Stationary Front
Nascent Stage
Mature Stage
Partially Occluded Stage
Occluded Stage
Dissipated Stage

10. Lifecycle of a Midlatitude Cyclone
Pressure surfaces tilt because of N-S temperature contrast
Passing wave initiates divergence and cyclonic vorticity
Cold air undercuts warm, and flows south
Cold air advection undermines upper trough, deepening it
N-S mixing in cyclone eventually consumes the available potential energy, and cyclone dies

11. Lifecycle of a Midlatitude Cyclone
Stationary front
Incipient cyclone
Open wave
Mature stage
occlusion
dissipating
Green shading indicates precipitation
Takes several days to a week, and moves 1000’s of km during lifecycle

12. Convergence and Divergence
Low
High
What initiates “cyclogenesis?”
When upper-level divergence is stronger than lower-level convergence, more air is taken out at the top than is brought in at the bottom. Surface pressure drops, and the low intensifies, or “deepens.”
500 mb height

14. Upper Air/Surface Relationship

16. Cyclone Development
baroclinic instability (baroclinic means temperature varies
on an isobaric surface) causes initial ‘perturbation’ to grow.
occurs in the presence of strong temperature gradients.
Imagine a short wave trough passes overhead (looking North):
Where will surface low develop?
Low
High
DIV
east

17. (looking North):
Near the surface, where will we have cold and warm
advection?
Will this amplify or weaken the upper level low?
How about the upper level divergence?
Will a more intense upper level low strengthen or weaken the
surface low?
High
High
Low
Low
DIV
DIV
warm
cool
Low
east

18. Passage of a shortwave often initiates the formation of a surface low.
Cyclone initiation
Passage of a shortwave often initiates the formation of a surface low.

19. Cyclone development:
Strong north south gradient+passage of a shortwave trough
Can lead to rapid cyclogenisis via baroclinic instability
(baroclinic means temperature varies on an isobaric surface)

20. Cyclone Development

21. What maintains the surface low?
Imagine a surface low forming directly below upper level low
Surface convergence “fills in” the low
Surface divergence “undermines” the high

22. Actual vertical structure
Upper level low is tilted westward with height with respect to the surface.
UPPER LEVEL DIVERGENCE INITIATES AND MAINTAINS A SURFACE LOW.

23. Baroclinic Instability
Upper level shortwave passes
Upper level divergence -> sfc low
Cold advection throughout lower troposphere
Cold advection intensifies upper low
Leads to more upper level divergence
Temperature advection is key!

24. Summary of Cyclone Weather
Roles of convergence and divergence aloft
Pattern of clouds, precipitation, and temperatures on the ground

25. Conveyor Belt Model
This model describes rising and sinking air along three “conveyor belts”
A warm conveyor belt rises with water vapor above the cold conveyor belt which also rises and turns.
Finally the dry conveyor belt descends bringing clearer weather behind the storm.

26. Questions to Think About
What is the vertical structure of a developing storm?
Where is the strongest upper/lower level divergence/convergence occurring?
Why aren’t the ‘lows’ vertically stacked?
What is required for a storm to develop?
Where is rising motion occurring?
Where is precipitation occurring?
What is the ultimate source of energy for a midlatitude storm?
Why does a storm “die”?
During what time of year would you expect more midlatitude cyclones?
Why doesn’t baroclinic instability occur in the tropics?

27. What is the source of energy for Midlatitude cyclones?
Potential energy arising from the temperature differences found in the different air masses.
Cold, dense air pushes warmer, less dense air up and out of the way.
“Up warm, down cold”

28. Development of surface cyclones is related to upper air wind patterns
Linked through the convergence /divergence patterns at the surface and aloft;
Convergence at the surface produces upward motion - divergence at the surface produces downward motion;
Convergence at upper levels produces downward motion - divergence produces upward motion

29. Poleward Energy Transport
MMC carries energy out of tropics (as gz)
Eddy motions carry it the rest of the way (mostly as transients)
Consider d/dj of energy flux (divergence)
Thermally indirect MMC in midlats is response to strong eddy flux divergences

30. Global Energy Cascade
Heterogeneous heating creates available potential energy (APE)
Thermally-direct MMC converts APE to zonal mean KE
Baroclinic instability converts zonal mean KE to eddy KE
Eddies shed eddies “and so on to viscosity”
Dissipation of KE converts energy back to heat

31. The “Big Picture”
We’ve emphasized horizontal transport of energy to balance the planetary energy budget:
Hadley Cell
Subtropical divergence
Midlatitude cyclones and conveyor belts
What about vertical motion?
“Up-warm, down cold”
“Up moist, down-dry”
Severe weather is all about vertical motion, and represents local release of energy that contributes to planetary energy balance